There are only really 3 websites worth considering
imdb does have misleading overall scores but does have the feature where you can view the score per demographic. This can be useful to see if a film gets an inflated score from say females under 18. The biggest problem it has is the scores are based on users and not critics. Users are notorious for rating films incorrectly, giving every film that they enjoy 10/10.
Rotten Tomatoes is probably the most accurate overall
Metacritic takes all the critics reviews and works out the average percentage giving more accurate scores. Rotten Tomatoes also does this and then uses it's weighted scoring system afterwards.
I would've said imdb, but since that's out of the question, the best one I can think of is Rotten Tomatoes.
Rotten Tomatoes is the most famous website and is known for correct ratings. Check it out.